Physical Geology Slides-Sedimentary Rocks

Landforms of Sedimentary Rocks

A mesa (Spanish for table) is a
flat-topped hill capped with resistant rocks (they can be
volcanic).

These mesas are in Canyonlands National
Park, Utah

Grandfather Bluff, near LaCrosse. Flat-topped
hill ... capped with resistant rocks ... what part of the
definition of "mesa" doesn't apply here?

Well,
none. If these were in Wyoming, we would call them mesas
without hesitating. We don't use the term because we
think of mesas as "western" and Wisconsin as
"eastern." But early writers on Wisconsin
geology called hills like this mesas without batting an
eye.

A ridge formed by steep resistant rocks
is a hogback. This hogback is in Wyoming.

When a mountain front is faced by steep
hogbacks, they are often called flatirons because,
well, they look like flatirons. These, the classic
examples, are near Boulder, Colorado.

Vertical beds can sometimes form steep
crags, like these at Garden of the Gods, Colorado Springs,
Colorado.

Stream Abrasion

Graded Bedding and Wave Action

This looks like horrible pollution but is actually
perfectly natural. On the Lake Michigan shoreline, wave
action washes away light minerals, leaving only black
magnetite sand at the high-water mark.

When wave conditions are right, layers of almost pure
magnetite can form, like those seen in this shallow
trench.

Cross-Beds and Channel Fill

Ripple Marks

Ripple marks on the muddy bottom of Green Bay, formed
during a low stand of the Bay.

Virtually identical ripple marks from the Baraboo
Quartzite, 1600 million years old.

When waves come from two directions at once,
interference ripple marks like these in Ontario form.

Modern interference ripple marks on the bottom of
Green Bay.

One way interference ripple marks can form.

Mud Cracks

Mud cracks on the bottom of a modern puddle.

Mud cracks in the dolomite at Maribel Caves, evidence
that the limy mud that formed these rocks occasionally
dried out. The rocks probably formed in a tidal flat
environment.

Mud cracks in Cambrian rocks in southwestern Virginia.
Even the curls of the flaking mud are preserved. We are
looking at the top of the beds here.

A Riddle

The inscriptions are not hard to figure out, but what
carved the perfectly circular hole?

Would it help to
note that Fort McCoy Military Reservation is just out of
the picture to the right?

A little search turned up the artifact that made the
hole. Humans affect the geologic record, and geologists
often have to decide if a feature is of natural or human
origin.

Loading Features

When sediment is deposited faster than it can compact,
it can sag downward into still-mushy layers below, and
mud can squirt upward. Sags like these are called load
casts. These are from the Antarctic Peninsula. Note
that the coarse bed is graded and the largest grains fill
the bottoms of the sags.

Sometimes mushy sediment will find only a few weak
spots, and produce isolated structures like this, called
a flame structure. This one is near Sudbury,
Ontario.

This looks like a dike, but close inspection shows
that the rock is sandstone. A clastic dike? Yes,
that's exactly what it's called. This one is north of
Lake Huron in Ontario. Clastic dikes form when sediment
is partially consolidated but under high pressure. If a
water-laden layer can find a weak spot in the overlying
layers, it squirts upward. Earthquakes are a common
trigger. Note how large pieces concentrated in the center
where flow was fastest.

Soft-Sediment Deformation

The very uniform layers here are varves,
alternating thick and thin layers formed in a glacial
lake. The contorted zone running across the center is
probably due to glacial ice shoving the upper layers of
sediment over the lower layers. Deformation that occurs
while sediments are partly or wholly unconsolidated is
called Soft-Sediment Deformation

Submarine Landslides

Submarine landslides are common on the edge of the
continental shelf. The landslide flows scour flutes in
the underlying sediment, then come to rest as graded beds
of sand and silt that fill in the sculpted bottom. We are
looking at the underside of a submarine landslide deposit
in New Brunswick.

Looking at the underside of another submarine
landslide deposit in New Brunswick. The flutes scoured by
submarine landslides tend to be steep on the upstream end
and gently-tapered on the downstream end, making it easy
to tell which way they moved. These structures are hard
to observe directly forming in nature but quite easy to
duplicate in the laboratory.

450 million years ago the continental margin of North
America was bordered by reefs much like the Great Barrier
Reef of Australia. Blocks of reef limestone broke off and
slid down the flanks of the reef to form this submarine
talus deposit, now exposed in Newfoundland. The layered
rocks at the bottom are part of a single block about 100
meters in size, surrounded on all sides by small rubble.